Traditional geothermal power generating systems are well known. Such systems, which are a renewable energy source with relatively low carbon emission footprints, are comprised of three primary designs, as follows:
(1) Dry steam power plants utilize steam (exiting wells drilled to a naturally occurring underground heat and steam production source) to drive a power generation turbine, with the turbine typically utilized to produce electricity. The areas in which a dry steam plant may be feasibly implemented, however, are relatively few.
(2) Flash steam geothermal power plants use water at very high pressure and temperature (i.e., at least approximately 182° C. or 360° F.) that is pumped under high pressure from typically very deep wells, to the surface. Upon reaching the surface, all or a portion of the very hot and high-pressure water is flashed into steam.
The steam drives a power generation turbine, again with the turbine typically producing electricity. The waste heat can be utilized for some alternative direct heat use, or for an additional binary power production means (as hereinafter explained), and/or is ultimately condensed in a cooling tower (or otherwise condensed) and returned to the sub-surface geology via a closed loop, an injection well, or the like.
The geographic locations with conditions suitable to pressurize water to an extent great enough to produce viable amounts of electrical power, however, are not common and are typically not economically available. This is because the temperature of the hot water exiting the well may need to exceed at least about 360° F. (as noted above), and in some cases may need to exceed about 600° F., for economically viable power production, and system implementation and operation is costly to access such high-level natural heat conditions.
For example, while 600° F. temperatures can be accessed most anywhere via drilling a deep enough well, such extremely deep wells may typically need to be about eight miles deep, as the temperature in the earth's crust reportedly increases approximately 72-81° F. per mile of depth from the surface. There are areas in the world where molten magma is found closer to the surface, and such areas are referred to as geothermal “hot spots”. However such “hot spot” areas are relatively rare.
Thus, to provide economically viable working pressure differentials for the turbine/generator situated between the geothermal heat source and the condenser (usually a cooling tower or the like) the water must be super-heated to a significant temperature typically found only at extreme depths. When one subjects a column of water to extreme depths, one increases the water pressure, which water pressure increases at about 0.433 pounds per square inch (“psi”) per foot of depth. This very high-pressure situation is why extremely hot water is circulated into and out of a very deep well absent ever boiling (the water pressure and the boiling point are both significantly elevated). Therefore, at least one (and typically two) power-consuming water circulating pump is/are necessary, among other equipment, for a flash steam power production plant. Also, known flash steam plants use water and therefore are limited to using the boiling/vaporization properties of water during system design.
(3) Binary Cycle Power Plants can utilize lower temperature ranges than common Flash Steam Plants to ultimately drive a power generation turbine, which is typically utilized to generate electricity. In a Binary Cycle Power Plant, geothermally heated hot water and/or steam, as well as waste heat from nuclear or fossil fuel power plants, or the like, is carried by a first primary water loop to heat and vaporize a secondary fluid within a secondary fluid loop, which secondary fluid has a boiling point lower than that of the hot water circulating within the first fluid loop. After heat is exchanged with the secondary loop, the water is re-circulated to the primary heat source to regain the maximum amount of heat possible so as to provide a continuous heat supply source for the secondary fluid loop.
The heat supplied to the primary water loop vaporizes and pressurizes the secondary fluid (typically a refrigerant with a lower boiling/vaporization point than water) within a vaporization chamber. The vaporized and pressurized secondary fluid then drives a turbine and/or generator. After exiting the turbine or generator, the secondary fluid is then condensed, typically by an air-source condenser and/or by nearby naturally occurring cool water, and is next re-introduced by a liquid pump into the vaporization chamber. As explained, the vaporization chamber's heat is derived from the first and primary water loop's absorbed heat from the primary geothermal or waste heat source.
An example of one low temperature heat source currently utilized for such a binary system is a hot spring in Alaska that produces water at approximately 165° F., with the system being proximate to a readily available, and naturally occurring, cold-water river. The cold-water river temperature (reportedly at about 40-50° F.) condenses the secondary fluid. Thus, in this particular application, the working temperature differential range may be approximately 115-125° F.
There are additional varying offshoot modes of geothermal power production plants. For example, some such plants utilize the heat available in hot (fully or partially molten) magma (in the 650° C. range), or in hot dry rocks, or in geysers, etc.
Thus, most conventional geothermal power production plants require somewhat uniquely occurring circumstances to be economically viable, typically require at least two independent fluid loops (with 2-3 liquid pumps) and are not universally available. Binary systems utilizing waste heat sources are more common, but either nuclear or fossil fuels are typically required as the primary heat source, both of which present environmental challenges.
Further, traditional Enhanced Geothermal Systems (EGS), using a binary geothermal system, have environmental challenges. EGS designs utilize geothermal heat as the heat source for the secondary closed refrigerant loop in a binary system to produce useable power. EGS designs typically acquire geothermal heat via circulating brine (salt water) through artificially created rock fractures between two deep wells (typically as much as 15,000 feet deep each). Regarding environmental challenges, EGS designs can create seismic activity when fracturing dry hot rocks between deep wells; can contribute to groundwater pollution when rock fracturing chemicals seep up into the aquifers; and lose significant quantities of water on a continuous basis in the open loop portion deep below the surface between the supply and return wells.
Despite the drawbacks of conventional geothermal power production plants, they provide more reliable and constant energy output than other types of renewable energy sources. Solar and wind, for example, are generally environmentally safe but are contingent upon uncontrollable conditions, such as the sun shining and the wind blowing. Accordingly, an interest remains in providing geothermal power production plants that are economically viable.